Gas

This articwe is about de physicaw properties of gas as a state of matter. For an automotive fuew, see autogas. For popuwar contraction for anoder automotive fuew, see gasowine. For de uses of gases, and oder meanings, see Gas (disambiguation).

Gas is one of de four fundamentaw states of matter (de oders being sowid, wiqwid, and pwasma). A pure gas may be made up of individuaw atoms (e.g. a nobwe gas wike neon), ewementaw mowecuwes made from one type of atom (e.g. oxygen), or compound mowecuwes made from a variety of atoms (e.g. carbon dioxide). A gas mixture wouwd contain a variety of pure gases much wike de air. What distinguishes a gas from wiqwids and sowids is de vast separation of de individuaw gas particwes. This separation usuawwy makes a coworwess gas invisibwe to de human observer. The interaction of gas particwes in de presence of ewectric and gravitationaw fiewds are considered negwigibwe, as indicated by de constant vewocity vectors in de image.

The gaseous state of matter is found between de wiqwid and pwasma states,[1] de watter of which provides de upper temperature boundary for gases. Bounding de wower end of de temperature scawe wie degenerative qwantum gases[2] which are gaining increasing attention, uh-hah-hah-hah.[3] High-density atomic gases super coowed to incredibwy wow temperatures are cwassified by deir statisticaw behavior as eider a Bose gas or a Fermi gas. For a comprehensive wisting of dese exotic states of matter see wist of states of matter.

The word gas was first used by de earwy 17f-century FwemishchemistJan Baptist van Hewmont.[4] He identified carbon dioxide, de first known gas oder dan air.[5] Van Hewmont's word appears to have been simpwy a phonetic transcription of de Ancient Greek word χάος Chaos – de g in Dutch being pronounced wike ch in "woch" (voicewess vewar fricative, IPA |x]) – in which case Van Hewmont was simpwy fowwowing de estabwished awchemicaw usage first attested in de works of Paracewsus. According to Paracewsus's terminowogy, chaos meant someding wike "uwtra-rarefied water".[6]

An awternative story[7] is dat Van Hewmont's word is corrupted from gahst (or geist), signifying a ghost or spirit. This was because certain gases suggested a supernaturaw origin, such as from deir abiwity to cause deaf, extinguish fwames, and to occur in "mines, bottom of wewws, churchyards and oder wonewy pwaces". In contrast, French-American historian Jacqwes Barzun[8] specuwated dat Van Hewmont had borrowed de word from de German Gäscht, meaning de frof resuwting from fermentation, uh-hah-hah-hah.

Gas particwes are widewy separated from one anoder, and conseqwentwy, have weaker intermowecuwar bonds dan wiqwids or sowids. These intermowecuwar forces resuwt from ewectrostatic interactions between gas particwes. Like-charged areas of different gas particwes repew, whiwe oppositewy charged regions of different gas particwes attract one anoder; gases dat contain permanentwy charged ions are known as pwasmas. Gaseous compounds wif powar covawent bonds contain permanent charge imbawances and so experience rewativewy strong intermowecuwar forces, awdough de mowecuwe whiwe de compound's net charge remains neutraw. Transient, randomwy induced charges exist across non-powar covawent bonds of mowecuwes and ewectrostatic interactions caused by dem are referred to as Van der Waaws forces. The interaction of dese intermowecuwar forces varies widin a substance which determines many of de physicaw properties uniqwe to each gas.[9][10] A comparison of boiwing points for compounds formed by ionic and covawent bonds weads us to dis concwusion, uh-hah-hah-hah.[11] The drifting smoke particwes in de image provides some insight into wow-pressure gas behavior.

Compared to de oder states of matter, gases have wow density and viscosity. Pressure and temperature infwuence de particwes widin a certain vowume. This variation in particwe separation and speed is referred to as compressibiwity. This particwe separation and size infwuences opticaw properties of gases as can be found in de fowwowing wist of refractive indices. Finawwy, gas particwes spread apart or diffuse in order to homogeneouswy distribute demsewves droughout any container.

When observing a gas, it is typicaw to specify a frame of reference or wengf scawe. A warger wengf scawe corresponds to a macroscopic or gwobaw point of view of de gas. This region (referred to as a vowume) must be sufficient in size to contain a warge sampwing of gas particwes. The resuwting statisticaw anawysis of dis sampwe size produces de "average" behavior (i.e. vewocity, temperature or pressure) of aww de gas particwes widin de region, uh-hah-hah-hah. In contrast, a smawwer wengf scawe corresponds to a microscopic or particwe point of view.

Macroscopicawwy, de gas characteristics measured are eider in terms of de gas particwes demsewves (vewocity, pressure, or temperature) or deir surroundings (vowume). For exampwe, Robert Boywe studied pneumatic chemistry for a smaww portion of his career. One of his experiments rewated de macroscopic properties of pressure and vowume of a gas. His experiment used a J-tube manometer which wooks wike a test tube in de shape of de wetter J. Boywe trapped an inert gas in de cwosed end of de test tube wif a cowumn of mercury, dereby making de number of particwes and de temperature constant. He observed dat when de pressure was increased in de gas, by adding more mercury to de cowumn, de trapped gas' vowume decreased (dis is known as an inverse rewationship). Furdermore, when Boywe muwtipwied de pressure and vowume of each observation, de product was constant. This rewationship hewd for every gas dat Boywe observed weading to de waw, (PV=k), named to honor his work in dis fiewd.

There are many madematicaw toows avaiwabwe for anawyzing gas properties. As gases are subjected to extreme conditions, dese toows become more compwex, from de Euwer eqwations for inviscid fwow to de Navier–Stokes eqwations[12] dat fuwwy account for viscous effects. These eqwations are adapted to de conditions of de gas system in qwestion, uh-hah-hah-hah. Boywe's wab eqwipment awwowed de use of awgebra to obtain his anawyticaw resuwts. His resuwts were possibwe because he was studying gases in rewativewy wow pressure situations where dey behaved in an "ideaw" manner. These ideaw rewationships appwy to safety cawcuwations for a variety of fwight conditions on de materiaws in use. The high technowogy eqwipment in use today was designed to hewp us safewy expwore de more exotic operating environments where de gases no wonger behave in an "ideaw" manner. This advanced maf, incwuding statistics and muwtivariabwe cawcuwus, makes possibwe de sowution to such compwex dynamic situations as space vehicwe reentry. An exampwe is de anawysis of de space shuttwe reentry pictured to ensure de materiaw properties under dis woading condition are appropriate. In dis fwight regime, de gas is no wonger behaving ideawwy.

The symbow used to represent pressure in eqwations is "p" or "P" wif SI units of pascaws.

When describing a container of gas, de term pressure (or absowute pressure) refers to de average force per unit area dat de gas exerts on de surface of de container. Widin dis vowume, it is sometimes easier to visuawize de gas particwes moving in straight wines untiw dey cowwide wif de container (see diagram at top of de articwe). The force imparted by a gas particwe into de container during dis cowwision is de change in momentum of de particwe.[13] During a cowwision onwy de normaw component of vewocity changes. A particwe travewing parawwew to de waww does not change its momentum. Therefore, de average force on a surface must be de average change in winear momentum from aww of dese gas particwe cowwisions.

Pressure is de sum of aww de normaw components of force exerted by de particwes impacting de wawws of de container divided by de surface area of de waww.

The symbow used to represent temperature in eqwations is T wif SI units of kewvins.

The speed of a gas particwe is proportionaw to its absowute temperature. The vowume of de bawwoon in de video shrinks when de trapped gas particwes swow down wif de addition of extremewy cowd nitrogen, uh-hah-hah-hah. The temperature of any physicaw system is rewated to de motions of de particwes (mowecuwes and atoms) which make up de [gas] system.[14] In statisticaw mechanics, temperature is de measure of de average kinetic energy stored in a particwe. The medods of storing dis energy are dictated by de degrees of freedom of de particwe itsewf (energy modes). Kinetic energy added (endodermic process) to gas particwes by way of cowwisions produces winear, rotationaw, and vibrationaw motion, uh-hah-hah-hah. In contrast, a mowecuwe in a sowid can onwy increase its vibrationaw modes wif de addition of heat as de wattice crystaw structure prevents bof winear and rotationaw motions. These heated gas mowecuwes have a greater speed range which constantwy varies due to constant cowwisions wif oder particwes. The speed range can be described by de Maxweww–Bowtzmann distribution. Use of dis distribution impwies ideaw gases near dermodynamic eqwiwibrium for de system of particwes being considered.

The symbow used to represent vowume in eqwations is "V" wif SI units of cubic meters.

When performing a dermodynamic anawysis, it is typicaw to speak of intensive and extensive properties. Properties which depend on de amount of gas (eider by mass or vowume) are cawwed extensive properties, whiwe properties dat do not depend on de amount of gas are cawwed intensive properties. Specific vowume is an exampwe of an intensive property because it is de ratio of vowume occupied by a unit of mass of a gas dat is identicaw droughout a system at eqwiwibrium.[15] 1000 atoms a gas occupy de same space as any oder 1000 atoms for any given temperature and pressure. This concept is easier to visuawize for sowids such as iron which are incompressibwe compared to gases. Since a gas fiwws any container in which it is pwaced, vowume is an extensive property.

The symbow used to represent density in eqwations is ρ (rho) wif SI units of kiwograms per cubic meter. This term is de reciprocaw of specific vowume.

Since gas mowecuwes can move freewy widin a container, deir mass is normawwy characterized by density. Density is de amount of mass per unit vowume of a substance, or de inverse of specific vowume. For gases, de density can vary over a wide range because de particwes are free to move cwoser togeder when constrained by pressure or vowume. This variation of density is referred to as compressibiwity. Like pressure and temperature, density is a state variabwe of a gas and de change in density during any process is governed by de waws of dermodynamics. For a static gas, de density is de same droughout de entire container. Density is derefore a scawar qwantity. It can be shown by kinetic deory dat de density is inversewy proportionaw to de size of de container in which a fixed mass of gas is confined. In dis case of a fixed mass, de density decreases as de vowume increases.

If one couwd observe a gas under a powerfuw microscope, one wouwd see a cowwection of particwes (mowecuwes, atoms, ions, ewectrons, etc.) widout any definite shape or vowume dat are in more or wess random motion, uh-hah-hah-hah. These neutraw gas particwes onwy change direction when dey cowwide wif anoder particwe or wif de sides of de container. In an ideaw gas, dese cowwisions are perfectwy ewastic. This particwe or microscopic view of a gas is described by de Kinetic-mowecuwar deory. The assumptions behind dis deory can be found in de postuwates section of Kinetic Theory.

Kinetic deory provides insight into de macroscopic properties of gases by considering deir mowecuwar composition and motion, uh-hah-hah-hah. Starting wif de definitions of momentum and kinetic energy,[16] one can use de conservation of momentum and geometric rewationships of a cube to rewate macroscopic system properties of temperature and pressure to de microscopic property of kinetic energy per mowecuwe. The deory provides averaged vawues for dese two properties.

The deory awso expwains how de gas system responds to change. For exampwe, as a gas is heated from absowute zero, when it is (in deory) perfectwy stiww, its internaw energy (temperature) is increased. As a gas is heated, de particwes speed up and its temperature rises. This resuwts in greater numbers of cowwisions wif de container per unit time due to de higher particwe speeds associated wif ewevated temperatures. The pressure increases in proportion to de number of cowwisions per unit time.

Brownian motion is de madematicaw modew used to describe de random movement of particwes suspended in a fwuid. The gas particwe animation, using pink and green particwes, iwwustrates how dis behavior resuwts in de spreading out of gases (entropy). These events are awso described by particwe deory.

Since it is at de wimit of (or beyond) current technowogy to observe individuaw gas particwes (atoms or mowecuwes), onwy deoreticaw cawcuwations give suggestions about how dey move, but deir motion is different from Brownian motion because Brownian motion invowves a smoof drag due to de frictionaw force of many gas mowecuwes, punctuated by viowent cowwisions of an individuaw (or severaw) gas mowecuwe(s) wif de particwe. The particwe (generawwy consisting of miwwions or biwwions of atoms) dus moves in a jagged course, yet not so jagged as wouwd be expected if an individuaw gas mowecuwe were examined.

As discussed earwier, momentary attractions (or repuwsions) between particwes have an effect on gas dynamics. In physicaw chemistry, de name given to dese intermowecuwar forces is van der Waaws force. These forces pway a key rowe in determining physicaw properties of a gas such as viscosity and fwow rate (see physicaw characteristics section). Ignoring dese forces in certain conditions awwows a reaw gas to be treated wike an ideaw gas. This assumption awwows de use of ideaw gas waws which greatwy simpwifies cawcuwations.

Proper use of dese gas rewationships reqwires de kinetic-mowecuwar deory (KMT). When gas particwes experience intermowecuwar forces dey graduawwy infwuence one anoder as de spacing between dem is reduced (de hydrogen bond modew iwwustrates one exampwe). In de absence of any charge, at some point when de spacing between gas particwes is greatwy reduced dey can no wonger avoid cowwisions between demsewves at normaw gas temperatures. Anoder case for increased cowwisions among gas particwes wouwd incwude a fixed vowume of gas, which upon heating wouwd contain very fast particwes. This means dat dese ideaw eqwations provide reasonabwe resuwts except for extremewy high pressure (compressibwe) or high temperature (ionized) conditions. Aww of dese excepted conditions awwow energy transfer to take pwace widin de gas system. The absence of dese internaw transfers is what is referred to as ideaw conditions in which de energy exchange occurs onwy at de boundaries of de system. Reaw gases experience some of dese cowwisions and intermowecuwar forces. When dese cowwisions are statisticawwy negwigibwe (incompressibwe), resuwts from dese ideaw eqwations are stiww meaningfuw. If de gas particwes are compressed into cwose proximity dey behave more wike a wiqwid (see fwuid dynamics).

An eqwation of state (for gases) is a madematicaw modew used to roughwy describe or predict de state properties of a gas. At present, dere is no singwe eqwation of state dat accuratewy predicts de properties of aww gases under aww conditions. Therefore, a number of much more accurate eqwations of state have been devewoped for gases in specific temperature and pressure ranges. The "gas modews" dat are most widewy discussed are "perfect gas", "ideaw gas" and "reaw gas". Each of dese modews has its own set of assumptions to faciwitate de anawysis of a given dermodynamic system.[17] Each successive modew expands de temperature range of coverage to which it appwies.

where P is de pressure, V is de vowume, n is amount of gas (in mow units), R is de universaw gas constant, 8.314 J/(mow K), and T is de temperature. Written dis way, it is sometimes cawwed de "chemist's version", since it emphasizes de number of mowecuwes n. It can awso be written as

P=ρRsT,{\dispwaystywe P=\rho R_{s}T,}

where Rs{\dispwaystywe R_{s}} is de specific gas constant for a particuwar gas, in units J/(kg K), and ρ = m/V is density. This notation is de "gas dynamicist's" version, which is more practicaw in modewing of gas fwows invowving acceweration widout chemicaw reactions.

The ideaw gas waw does not make an assumption about de specific heat of a gas. In de most generaw case, de specific heat is a function of bof temperature and pressure. If de pressure-dependence is negwected (and possibwy de temperature-dependence as weww) in a particuwar appwication, sometimes de gas is said to be a perfect gas, awdough de exact assumptions may vary depending on de audor and/or fiewd of science.

For an ideaw gas, de ideaw gas waw appwies widout restrictions on de specific heat. An ideaw gas is a simpwified "reaw gas" wif de assumption dat de compressibiwity factorZ is set to 1 meaning dat dis pneumatic ratio remains constant. A compressibiwity factor of one awso reqwires de four state variabwes to fowwow de ideaw gas waw.

This approximation is more suitabwe for appwications in engineering awdough simpwer modews can be used to produce a "baww-park" range as to where de reaw sowution shouwd wie. An exampwe where de "ideaw gas approximation" wouwd be suitabwe wouwd be inside a combustion chamber of a jet engine.[18] It may awso be usefuw to keep de ewementary reactions and chemicaw dissociations for cawcuwating emissions.

Each one of de assumptions wisted bewow adds to de compwexity of de probwem's sowution, uh-hah-hah-hah. As de density of a gas increases wif rising pressure, de intermowecuwar forces pway a more substantiaw rowe in gas behavior which resuwts in de ideaw gas waw no wonger providing "reasonabwe" resuwts. At de upper end of de engine temperature ranges (e.g. combustor sections – 1300 K), de compwex fuew particwes absorb internaw energy by means of rotations and vibrations dat cause deir specific heats to vary from dose of diatomic mowecuwes and nobwe gases. At more dan doubwe dat temperature, ewectronic excitation and dissociation of de gas particwes begins to occur causing de pressure to adjust to a greater number of particwes (transition from gas to pwasma).[19] Finawwy, aww of de dermodynamic processes were presumed to describe uniform gases whose vewocities varied according to a fixed distribution, uh-hah-hah-hah. Using a non-eqwiwibrium situation impwies de fwow fiewd must be characterized in some manner to enabwe a sowution, uh-hah-hah-hah. One of de first attempts to expand de boundaries of de ideaw gas waw was to incwude coverage for different dermodynamic processes by adjusting de eqwation to read pVn = constant and den varying de n drough different vawues such as de specific heat ratio, γ.

Reaw gas effects incwude dose adjustments made to account for a greater range of gas behavior:

For most appwications, such a detaiwed anawysis is excessive. Exampwes where reaw gas effects wouwd have a significant impact wouwd be on de Space Shuttwere-entry where extremewy high temperatures and pressures were present or de gases produced during geowogicaw events as in de image of de 1990 eruption of Mount Redoubt.

Boywe's Law was perhaps de first expression of an eqwation of state. In 1662 Robert Boywe performed a series of experiments empwoying a J-shaped gwass tube, which was seawed on one end. Mercury was added to de tube, trapping a fixed qwantity of air in de short, seawed end of de tube. Then de vowume of gas was carefuwwy measured as additionaw mercury was added to de tube. The pressure of de gas couwd be determined by de difference between de mercury wevew in de short end of de tube and dat in de wong, open end. The image of Boywe's Eqwipment shows some of de exotic toows used by Boywe during his study of gases.

Through dese experiments, Boywe noted dat de pressure exerted by a gas hewd at a constant temperature varies inversewy wif de vowume of de gas.[20] For exampwe, if de vowume is hawved, de pressure is doubwed; and if de vowume is doubwed, de pressure is hawved. Given de inverse rewationship between pressure and vowume, de product of pressure (P) and vowume (V) is a constant (k) for a given mass of confined gas as wong as de temperature is constant. Stated as a formuwa, dus is:

PV=k{\dispwaystywe PV=k}

Because de before and after vowumes and pressures of de fixed amount of gas, where de before and after temperatures are de same bof eqwaw de constant k, dey can be rewated by de eqwation:

In 1787, de French physicist and bawwoon pioneer, Jacqwes Charwes, found dat oxygen, nitrogen, hydrogen, carbon dioxide, and air expand to de same extent over de same 80 kewvin intervaw. He noted dat, for an ideaw gas at constant pressure, de vowume is directwy proportionaw to its temperature:

In 1802, Joseph Louis Gay-Lussac pubwished resuwts of simiwar, dough more extensive experiments.[21] Gay-Lussac credited Charwes' earwier work by naming de waw in his honor. Gay-Lussac himsewf is credited wif de waw describing pressure, which he found in 1809. It states dat de pressure exerted on a container's sides by an ideaw gas is proportionaw to its temperature.

In 1811, Amedeo Avogadro verified dat eqwaw vowumes of pure gases contain de same number of particwes. His deory was not generawwy accepted untiw 1858 when anoder Itawian chemist Staniswao Cannizzaro was abwe to expwain non-ideaw exceptions. For his work wif gases a century prior, de number dat bears his name Avogadro's constant represents de number of atoms found in 12 grams of ewementaw carbon-12 (6.022×1023 mow−1). This specific number of gas particwes, at standard temperature and pressure (ideaw gas waw) occupies 22.40 witers, which is referred to as de mowar vowume.

Avogadro's waw states dat de vowume occupied by an ideaw gas is proportionaw to de number of mowes (or mowecuwes) present in de container. This gives rise to de mowar vowume of a gas, which at STP is 22.4 dm3 (or witres). The rewation is given by

In 1801, John Dawton pubwished de Law of Partiaw Pressures from his work wif ideaw gas waw rewationship: The pressure of a mixture of non reactive gases is eqwaw to de sum of de pressures of aww of de constituent gases awone. Madematicawwy, dis can be represented for n species as:

Pressuretotaw = Pressure1 + Pressure2 + ... + Pressuren

The image of Dawton's journaw depicts symbowogy he used as shordand to record de paf he fowwowed. Among his key journaw observations upon mixing unreactive "ewastic fwuids" (gases) were de fowwowing:[22]

Thermodynamicists use dis factor (Z) to awter de ideaw gas eqwation to account for compressibiwity effects of reaw gases. This factor represents de ratio of actuaw to ideaw specific vowumes. It is sometimes referred to as a "fudge-factor" or correction to expand de usefuw range of de ideaw gas waw for design purposes. Usuawwy dis Z vawue is very cwose to unity. The compressibiwity factor image iwwustrates how Z varies over a range of very cowd temperatures.

In fwuid mechanics, de Reynowds number is de ratio of inertiaw forces (vsρ) to viscous forces (μ/L). It is one of de most important dimensionwess numbers in fwuid dynamics and is used, usuawwy awong wif oder dimensionwess numbers, to provide a criterion for determining dynamic simiwitude. As such, de Reynowds number provides de wink between modewing resuwts (design) and de fuww-scawe actuaw conditions. It can awso be used to characterize de fwow.

Viscosity, a physicaw property, is a measure of how weww adjacent mowecuwes stick to one anoder. A sowid can widstand a shearing force due to de strengf of dese sticky intermowecuwar forces. A fwuid wiww continuouswy deform when subjected to a simiwar woad. Whiwe a gas has a wower vawue of viscosity dan a wiqwid, it is stiww an observabwe property. If gases had no viscosity, den dey wouwd not stick to de surface of a wing and form a boundary wayer. A study of de dewta wing in de Schwieren image reveaws dat de gas particwes stick to one anoder (see Boundary wayer section).

In fwuid dynamics, turbuwence or turbuwent fwow is a fwow regime characterized by chaotic, stochastic property changes. This incwudes wow momentum diffusion, high momentum convection, and rapid variation of pressure and vewocity in space and time. The satewwite view of weader around Robinson Crusoe Iswands iwwustrates one exampwe.

Particwes wiww, in effect, "stick" to de surface of an object moving drough it. This wayer of particwes is cawwed de boundary wayer. At de surface of de object, it is essentiawwy static due to de friction of de surface. The object, wif its boundary wayer is effectivewy de new shape of de object dat de rest of de mowecuwes "see" as de object approaches. This boundary wayer can separate from de surface, essentiawwy creating a new surface and compwetewy changing de fwow paf. The cwassicaw exampwe of dis is a stawwing airfoiw. The dewta wing image cwearwy shows de boundary wayer dickening as de gas fwows from right to weft awong de weading edge.

As de totaw number of degrees of freedom approaches infinity, de system wiww be found in de macrostate dat corresponds to de highest muwtipwicity. In order to iwwustrate dis principwe, observe de skin temperature of a frozen metaw bar. Using a dermaw image of de skin temperature, note de temperature distribution on de surface. This initiaw observation of temperature represents a "microstate". At some future time, a second observation of de skin temperature produces a second microstate. By continuing dis observation process, it is possibwe to produce a series of microstates dat iwwustrate de dermaw history of de bar's surface. Characterization of dis historicaw series of microstates is possibwe by choosing de macrostate dat successfuwwy cwassifies dem aww into a singwe grouping.

When energy transfer ceases from a system, dis condition is referred to as dermodynamic eqwiwibrium. Usuawwy, dis condition impwies de system and surroundings are at de same temperature so dat heat no wonger transfers between dem. It awso impwies dat externaw forces are bawanced (vowume does not change), and aww chemicaw reactions widin de system are compwete. The timewine varies for dese events depending on de system in qwestion, uh-hah-hah-hah. A container of ice awwowed to mewt at room temperature takes hours, whiwe in semiconductors de heat transfer dat occurs in de device transition from an on to off state couwd be on de order of a few nanoseconds.

^One noticeabwe exception to dis physicaw property connection is conductivity which varies depending on de state of matter (ionic compounds in water) as described by Michaew Faraday in 1833 when he noted dat ice does not conduct a current. See page 45 of John Tyndaww's Faraday as a Discoverer (1868).